Went with Santander's original rate (5)
'rate' is the definition.
(speed is a kind of rate)
'went with santander's original' is the wordplay.
'went' becomes 'peed' (both can mean urinated).
'with' says to put letters next to each other.
'original' says to take the initial letters.
The initial letter of 'santander' is 's'.
'peed' after 's' is 'SPEED'.
(Other definitions for speed that I've seen before include "Swiftness" , "Hasten" , "Pace or tempo" , "Rate of progress" , "Move quickly (and illegally?)" .)
But I've got no explanation for this one! Could you help me understand?
(Other definitions for spend that I've seen before include "Part with money" , "Use up (one's resources)" , "Pay out or use up" , "Use (money etc)" , "Blow" .)